Polymeric microporous materials have had a great impact on academic research and industrial applications. To date, several types of microporous polymeric materials have been reported, for example solvent swollen crosslinked polymers (e.g., hypercrosslinked polystyrenes) [Davankov 1990; Tsyurupa 2002], rigid polymer networks [Budd 2003; McKeown 2006b; Webster 1992; Urban 1995; Wood 2007; McKeown 2002], rigid non-network polymers such as poly(1-trimethylsilyl-1-propyne) [Masuda 1983; Nagai 2001], certain polyimides [Tanaka 1992; Weber 2007], and a number of fluorinated polymers [Yu 2002] or polymers with bulky structural units [Dai 2004; Dai 2005]. Such microporous materials are of potential use in applications such as adsorbents, separation materials, and catalysis, since they combine high internal surface area (comparable with conventional microporous materials, such as zeolites or activated carbons) with the processability of polymers.
Polymer membrane gas separation is a dynamic and rapidly growing field of separation technology [Stern 1994; Maier 1998] because it can offer a number of advantages, such as low energy use and capital cost [Pandey 2001]. In recent years, much effort has been devoted to the design and preparation of membrane materials whose transport properties are improved by overcoming the “trade-off” behavior between permeability and selectivity [Kim 1988; Lee 1988; Robeson 1991; Robeson 1994].
Recently, Budd and co-workers described a novel class of high-free volume polymeric microporous materials derived from nitrile monomers termed “polymers of intrinsic microporosity” (PIMs) whose rigid and randomly contorted structures increase high-free volume and surface area while decreasing chain packing efficiently and pore collapse in the solid state [Budd 2004a; Budd 2004b; Budd 2005a; Budd 2005b; McKeown 2005]. Compared to conventional gas separation polymers, the profound significance of these polymers is that they simultaneously display both very high gas permeability and good selectivity, contrary to the normal trade-off behavior of many traditional thermoplastic polymers. These microporous materials are soluble in several common solvents and can be readily fabricated into thin films. Consequently, they have attracted great interest as outstanding membrane materials which have a high potential for gas separation [Budd 2004b; Budd 2005b], adsorption of small molecules such as hydrogen [McKeown 2006b; Ghanem 2007; Budd 2007; McKeown 2007], heterogeneous catalysis [Budd 2003; McKeown 2006c] and as adsorbents for organic compounds [Budd 2003; Maffei 2006].
One important structural feature of PIMs is the presence of kinks in the repeat units. For example, PIM-1, the most studied PIM having a high molecular weight, is prepared from a dioxane-forming reaction between commercially available 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethylspirobisindane (TTSBI), and 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN). Although McKeown and Budd suggested several compounds which include a spiro-contorted site as PIMs monomers [McKeown 2006a] there are only a few such monomers reported that provide PIMs for which gas permeabilities have been measured [Ghanem 2008; Carta 2008; Kricheldorf 2006; Budd 2004a]. These few PIMs have been synthesized using a controlled low temperature aromatic nucleophilic substitution polycondensation of tetraphenol monomers with tetrahalogenated monomers containing nitrile or imine electron-withdrawing groups. Among these polymers, they reported the gas permeability coefficients of some ladder polymers such as PIM-1 and PIM-7. PIM-1, is prepared from commercially available 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethylspirobisindane (TTSBI) and 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN) by an efficient double aromatic nucleophilic substitution (SNAr) polycondensation.
Few PIM structures with high molecular weight have been reported to date due to the few choices of available monomers and the inability of available monomers to produce sufficiently high molecular weight polymers. The latter is an important consideration in using these materials for membrane gas separation, where materials with high molecular weight are required for coating onto supports and for fabricating free-standing films.
As is well known, the chemical structure and physical properties of membrane materials influence permeability and selectivity [Pandey 2001; Aoki 1999; Dai 2004; George 2001]. Many studies have shown that an improvement in gas transport properties could be obtained by modifying or tailoring the polymer structure. Considerable attention has been devoted to the preparation of new classes of partially fluorinated polymers because of their unusual properties. Trifluoromethyl groups (—CF3) have been reported to significantly improve permeability and selectivity by increasing chain stiffness and reducing interchain interactions such as charge-transfer complexes (CTCs) [Banerjee 1999; Dai 2005]. In addition, —CF3 groups in a polymer backbone serve several other purposes, such as enhancing polymer solubility (commonly referred to as the fluorine effect) without forfeiting thermal stability, lowering dielectric constants and water absorption, increasing the fractional free volume (FFV) of polymers, and increasing glass transition temperature (Tg) with concomitant decrease and/or elimination of crystallinity. The phenylsulfonyl group (—SO2C6H5) is also a useful group which is employed beneficially in polymers used for gas separation. In general, the sulfonyl group (—SO2—) raises Tg through increasing rigidity of the polymer chain and reduces FFV and permeability, while increasing selectivity [Paul 1994].
Processes for producing PIMs like those produced by Budd and coworkers have been studied under seemingly similar reaction conditions (Kricheldorf 2004). Kricheldorf and coworkers concluded that the majority of the product was cyclic which results in low molecular weight polymer and high polydispersity indices. Further, they found that the use of high temperature or high concentration of reactants, which have previously been shown to favor the decrease of cyclic oligomers, cannot be applied in this reaction due to explosive polycondensation yielding cross-linked product. It is well known that the rate-controlling step in this polycondensation reaction is the dissolution of the monomer salt. The cyclic compounds were formed in the reaction mixture as a result of the high dilution conditions created by poor solubility of the salt. Further, cyclization competes with every chain-growth step at all stages of polycondensation. Further, it has been observed that crosslinking happened quickly when the polymer precipitated from the reaction mixture. Therefore, it is of importance to develop an efficient polycondensation method for preparing PIMs that are substantially free of cyclics and crosslinked structures.
Thus, there is a need in the art to expand the spectrum of high molecular weight PIMs having new structures derived from different monomers for use in membranes having improved gas permeability and separation properties. There is also a need for more efficient processes for producing such polymers.